1. Field of the Invention
This invention relates to an electrophotographic method and apparatus, and more particularly to a method and apparatus which can achieve good half-tone representation of monochromatic and multi-colored images.
2. Description of the Prior Art
Projection of an image divided into the form of meshes has heretofore been adopted to provide a good half-tone representation. Division of an image into such mesh form has usually been done by disposing a grid between an original to be copied and a glass plate supporting the original thereon or by providing the glass plate itself with grid lines.
The grid of this type must have a very large size, i.e. a size corresponding to that of the glass plate supporting the original. Formation of such grid is quite expensive.
Further, where a colored copy is to be produced, the grid must be rotated when each color-resolved image is projected. Otherwise, the resultant copy would exhibit a ripply pattern (moire stripes).
On the other hand, the original being copied must be maintained accurately in its position until each individual color-resolved image is formed and moreover, these individual color-resolved images must be projected in sufficiently accurate superposed relationship. These relationships are difficult to maintain when the grid is rotated. Further, the original to be copied is usually rectangular in shape and therefore, the grid must have a large surface area sufficient to fully cover the surface of the original in any of the successive rotated positions of grid. Accordingly, a large space must be provided for the grid.
There has also been proposed a method and apparatus which uses a photosensitive medium formed with a mesh-like pattern, instead of a grid formed in the original carrier. However, this is unsuitable and inconvenient for color representation because the mesh-like pattern is immovable during the color representation and the creation of moire stripes is unavoidable.
One of the methods heretofore employed for color representation is to repeatedly expose a sheet belt of zinc oxide to various color-resolved images of an original. During the interval between the individual exposure steps, the image is developed by means of a corresponding color developer and the copy paper is dried prior to the next charging step.
Another conventional method is to repeatedly expose a photosensitive medium disposed on the cylindrical surface of a drum to various color-resolved images of an original. In such cyclical process wherein charging, exposure, development, etc. are repeated to represent each resolved color, synchronism between conveyance of the copy paper and rotational speed of the drum is indispensable during the image transfer step.
Production of colored copies requires a number of steps to be repeated and the control mechanism therefor becomes complicated.
In achieving the above-described mesh-like image exposure, the conventional methods and apparatuses require additional steps and involve complicated construction as compared to the present invention.